TERRESTRIAL VISITORS

Q&A: Barometric Air Pressure

July 22, 2013

Question: The local weatherman always reports something called “barometric pressure” but I have no idea what he’s talking about. Whatever the reported pressure, I never feel any difference. What exactly is barometric pressure, and why should I care whether it’s high or low? — TJR, Birch Lake, Wisconsin

Answer: Barometric pressure is often measured in units of millibars (mb). 1 mb = 1/1000 bar, where 1 bar = 100,000 N/m2 (newtons per square meter). 1 bar is also (approximately) the average atmospheric pressure at sea level. All these units of measurement are derived from the metric system (SI). Using imperial units, 1000 mb = 14.5 psi (pounds per square inch). The term “bar” comes from the Greek word “baros,” meaning “weight.”

You’ll also sometimes hear barometric pressure expressed in millimeters (or inches). This is because barometric pressure was traditionally measured using a mercury-filled device called a barometer. Using these alternate units of measurement, average atmospheric pressure at sea level is around 750 mm-Hg (29.5 in-Hg). That number measures the height of a vertical column of mercury inside the device.

However you choose to measure it, this pressure is a result of the weight of the air in a column extending from sea level to an altitude of 100 km (62 miles), which is where the atmosphere effectively ends and the vacuum of outer space begins. You don’t normally feel it, since the same pressure exists inside your body. But if you change altitude rapidly, or if there’s a significant change in pressure at ground level, you can often feel it in your sinuses, or your ears might “pop” as your internal body pressure attempts to equalize with the ambient air pressure.

By comparison, if you are under water (which is much heavier than air), a depth of only 10 meters (33 feet) will produce the same pressure as 100 km of air. This why scuba divers require pressurized air to inflate their lungs for breathing.

For weather purposes, the numerical value for the barometric pressure is less important than whether it’s increasing or decreasing, both of which portend a change in weather conditions due to an approaching high or low pressure system — those are the big H‘s and L‘s you always see on weather maps. Normal highs and lows are typically only a few percent different from the average atmospheric pressure.

Finally, I should distinguish between gauge pressure and absolute pressure, the latter of which is used for expressing atmospheric pressure. Gauge pressure is the difference between inside and outside pressure. If you have a totally flat tire, its gauge pressure will measure zero even though the absolute pressure inside the tire equals the ambient atmospheric air pressure. That doesn’t help much in terms of mobility, but just be glad the absolute pressure isn’t zero. In that case, you’d have a lot more to worry about than a flat.

And I find your question very interesting. Bottom line: How does an altimeter “know” whether the 950 mb reading it senses is related to altitude or a low pressure weather system or both? There are answers, but you’ll have to wait till Aug 26 to read them. Just now plugged your question into my queue as “How Altimeters Measure Altitude.”